A typical aircraft has landing gear comprising a plurality of undercarriages which support the aircraft when it is on the ground. The undercarriages are used to control the movement of the aircraft during ground manoeuvres such as landing, taxiing and take off. Some of the undercarriages have braking wheels which are operable to provide a braking force to decelerate the aircraft when a braking torque is applied by a set of brakes. One of the undercarriages has a steering wheel which may be rotated to steer the aircraft.
During some ground manoeuvres, the steering wheel may start to skid when it is rotated to steer the aircraft, particularly if the aircraft is operated during adverse runway conditions. If the steering wheel starts to skid, its ability to generate lateral forces to turn the aircraft is reduced, therefore reducing the turning performance of the aircraft. Corrective action may be required to reduce the steer angle of the steering wheel and reduce skidding, increasing the workload of the pilot and/or control systems controlling the steering wheel.
During some ground manoeuvres, the aircraft may experience unintended changes in direction, for example due to external factors such as cross-winds, gusts and variations in a runway surface, as well as aircraft factors such as uneven tyre pressure, asymmetric braking, asymmetric engine thrust or component failures. Asymmetric braking refers to an unintentional difference in braking forces generated either side of an aircraft centre line resulting in an unintended yaw moment If an aircraft experiences an unintended change in direction, corrective action may be required to restore the aircraft to the desired heading, increasing the workload of the pilot and/or control systems controlling the steering wheel.
Aircraft undercarriages are typically designed conservatively to withstand significant variations in loading during ground manoeuvres. This conservative design can result in a significant weight penalty, therefore decreasing aircraft fuel efficiency.
It is therefore desirable to provide a steering system for an aircraft which addresses these problems and enables an aircraft to perform ground manoeuvres with maximum efficiency within a prescribed performance envelope.